A switching circuit used for a DC-DC converter and the like alternately turns on/off, according to a control signal, two switching elements connected in series between both ends of a DC power source, to supply power to a load connected at an intermediate point of the two switching elements. To drive the switching element connected to a high side of the DC power source, the switching circuit requires a dedicated power source. To form this power source, conventionally known is a drive circuit using a bootstrap circuit.
FIG. 1 is a circuit diagram showing a drive circuit using a bootstrap circuit. In FIG. 1, a MOSFET as a switching element QH and a MOSFET as a switching element QL are connected in series between both ends of a DC power source Vin. A connection point of the switching element QH and switching element QL is connected to a load 80.
A low-side drive circuit 60 turns on/off the switching element QL according to a signal from a control circuit that is not shown. According to a signal from the not-shown control circuit, a high-side drive circuit 70 turns on/off the switching element QH complementarily to the switching element QL. Namely, the control signals alternately turn on/off the switching element QH and switching element QL.
The low-side drive circuit 60 operates with an auxiliary power source Vcc1. The high-side drive circuit 70 operates with the voltage of a capacitor C2 that is charged by the bootstrap circuit 50 including a diode D1 and the capacitor C2.
Operation of the drive circuit of such a configuration will be explained. Here, charging the capacitor C2 serving as a power source for operating the high-side drive circuit 70 will be explained.
A control signal turns on the switching element QL, and then, the auxiliary power source Vcc1 passes a current through a path along the diode D1, the capacitor C2, the connection point V0 of the switching elements QH and QL, the switching element QL, and a ground GND, to charge the capacitor C2.
Then, the control signal turns off the switching element QL to increase a potential at the connection point V0. This increases a reference potential for the capacitor C2 and high-side drive circuit 70 and the voltage of the charged capacitor C2 operates the high-side drive circuit 70.
FIG. 2 is a timing chart explaining operation of the drive circuit shown in FIG. 1. FIG. 2(a) shows a voltage V0-GND between the connection point V0 and the ground GND and FIG. 2(b) shows a voltage VB-V0 of the capacitor C2. When the switching elements QH and QL are in an OFF state, the voltage V0-GND is about a half the DC power source voltage Vin due to division by floating capacitance of the switching elements and the like. The capacitor C2 has no charge, and therefore, the voltage VB-V0 is nil.
At time t11, the switching element QL turns on and the voltage V0-GND becomes nearly zero. At this time, the capacitor C2 is charged by the auxiliary power source Vcc1 through the path along the diode D1, capacitor C2, connection point V0 of the switching elements QH and QL, switching element QL, and ground GND, to rapidly increase the potential thereof. At time t12, the capacitor C2 is charged to a voltage equal to the voltage of the auxiliary power source Vcc1.
At time t13, the switching element QL turns off and the high-side drive circuit 70 that is operable with the voltage VB-V0 of the capacitor C2 turns on the switching element QH. As a result, the voltage V0-GND increases to the input voltage Vin and the voltage VB-V0 of the capacitor C2 decreases because the charge of the capacitor C2 is consumed by the high-side drive circuit 70 to turn on the switching element QH.
At time t14, the switching element QH turns off and the switching element QL turns on to again charge the capacitor C2. These actions repeat to alternately turn on/off the switching elements QH and QL.
A gate drive circuit related to the drive circuit shown in FIG. 1 is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2004-304527. The gate drive circuit described in this document combines a bootstrap circuit with a charge pump drive circuit to provide a power source for a drive circuit of a high-side switching element.
FIG. 3 is a circuit diagram showing the gate drive circuit of the related art. Operation of the gate drive circuit will be explained. When a lower-arm switching element 122 turns on, a bootstrap charge circuit is formed. Namely, a power source capacitor 110 of an upper arm circuit is charged through a charging path extending along a power source 106, high-voltage diodes 111 and 112, the power source capacitor 110 of the upper arm circuit, a connection point A of the switching elements 121 and 122, the lower-arm switching element 122, and a ground.
When the upper-arm switching element 121 turns on, a potential at the point A increases nearly to the voltage of a DC power source 101. At this time, an oscillation circuit 113 operates. According to an output from the oscillation circuit 113, a charge pump drive circuit 116 alternately turns on/off switching elements 117 and 118. At ON-timing of the switching element 118, the power source 106 charges an auxiliary capacitor 119 through a path extending along the diode 111, auxiliary capacitor 119, and switching element 118. Then, the switching element 118 turns off and the switching element 117 turns on. The charged auxiliary capacitor 119 charges the power source capacitor 110 of the upper-arm circuit through a path extending along the high-voltage diode 112, power source capacitor 110, and switching element 117.
In this way, when the low-side switching element 122 is OFF, the charge pump drive circuit 116 charges the power source capacitor 110 of the bootstrap circuit.